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Security researchers have found an important missing piece in the Stuxnet jigsaw that provides evidence that the malware was targeted at the types of control systems more commonly found in nuclear plants and other specialised operations than in mainstream factory controls.

It was already known that the highly sophisticated Stuxnet worm targets industrial plant control (SCADA) systems from Siemens, spreading using either unpatched Windows vulnerabilities or from infected USB sticks. The malware only uses infected PCs as a conduit onto connected industrial control systems. The malware is capable of reprogramming or even sabotaging targeted systems while hiding its presence using rootkit-style functionality.

New research, published late last week, has established that Stuxnet searches for frequency converter drives made by Fararo Paya of Iran and Vacon of Finland. In addition, Stuxnet is only interested in frequency converter drives that operate at very high speeds, between 807 Hz and 1210 Hz.

The malware is designed to change the output frequencies of drives, and therefore the speed of associated motors, for short intervals over periods of months. This would effectively sabotage the operation of infected devices while creating intermittent problems that are that much harder to diagnose.

Low-harmonic frequency converter drives that operate at over 600 Hz are regulated for export in the US by the Nuclear Regulatory Commission as they can be used for uranium enrichment. They may have other applications but would certainly not be needed to run a conveyor belt at a factory, for example.

Symantec - which has an informative write-up piece here - describes the new research as a "critical piece of the puzzle". Eric Chien, a senior researcher at Symantec, writes. "With this discovery, we now understand the purpose of all of Stuxnet’s code".

Although we know what Stuxnet does, we still can't be sure who created it or its exact purpose, although we can make an educated guess.

Stuxnet infections first surfaced in Malaysia in June, but the appearance of the malware in Iran has long been the major point of interest in the story. Plant officials at the controversial Bushehr nuclear plant in Iran admitted the malware had infected its network in September. This had nothing to do with a recently announced two-month delay in bringing the reactor online, government ministers subsequently claimed.

One theory is that Russian contractors at the site of Bushehr power plant introduced the malware, either accidentally or (more likely) deliberately. Stuxnet used four Windows zero-day vulnerabilities to spread and must have been developed by a team with expertise in and access to industrial control systems over several weeks, at a minimum. Altogether an expensive and tricky project with no obvious financial return, factors suggest the malware was developed with either the direct involvement of support of intelligence agencies or nation-states and designed for sabotage.

The appearance of the malware has provoked talk of cyberwar in some quarters and certainly done a great deal to raise the profile of potential attacks on power grid and utility systems in the minds of politicians. This is regardless of the potential likelihood of such an attack actually being successful, which remains unclear even after the arrival of Stuxnet. ®